Plasma processing apparatus with reduced parasitic capacity and loss in RF power

ABSTRACT

A plasma processing apparatus comprises a grounded housing, a thin RF plate electrode, an opposite electrode facing the RF plate electrode, and a RF power source for applying a radio frequency to either the RF plate electrode or the opposite electrode to produce plasma between the two electrodes. If the radio frequency applied to the electrode is f (MHz), the parasitic capacity C (pF) between the grounded portion of the housing and a conductive portion through which the radio frequency propagates is less than 1210*f −0.9 . The thickness of the RF plate electrode is 1 mm to 6 mm, and it is supported by a heat sink. The heat sink has a coolant passage in the proximity to the RF plate electrode. The heat sink also has a groove or a cavity in addition to the coolant passage, thereby reducing the value of the dielectric constant of the heat sink as a whole.

[0001] The present patent application claims the benefit of earlierJapanese Patent Application No. 2000-195165 filed Jun. 28, 2000, thedisclosure of which is herein incorporated entirely by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a plasma processing apparatusfor producing plasma under application of a radio frequency and forcarrying out etching or CVD processes.

[0004] 2. Description of the Related Art

[0005] A parallel plate type plasma etching apparatus is generally usedin semiconductor manufacturing processes. In the conventional plasmaetching apparatus, a radio frequency of about 13.56 MHz is applied tothe cathode electrode to excite plasma. However, in order to keep upwith increasingly strict design rules, and in order to respond to ademand for improvements in productivity, techniques of applying a higherrange of radio frequency, e.g., the frequency band of VHF to UHF havebeen studied. The proposal of raising the radio frequency applied to theelectrode also responds to a demand for an increase in wafer size, inwhich more intricate patterns are to be formed.

[0006] However, as the frequency applied to the electrode becomeshigher, the loss of RF power also increases. If the loss of RF powerincreases, the electron density of plasma produced between the parallelplate electrodes decreases, lowering the etching rate. Consequently, awafer cannot be precisely etched into a designed shape and pattern.

[0007]FIG. 1 illustrates a conventional plasma processing apparatus. Theconventional apparatus comprises an RF electrode 4, to which an RF isapplied, and a metallic DC plate 2, to which a direct-current voltage isapplied, a ceramic or resin insulator 3 a, and an opposite electrode 5facing the RF electrode. The conventional RF electrode 4 is maderelatively thick because the RF electrode itself has a wafer supportfunction. The RF electrode 4, the DC plate 2, and the insulator 3 a and3 b constitute a wafer hold structure.

[0008] As illustrated in FIG. 2, pusher pins 12 extend penetrating theRF electrode 4, the DC plate 2 and the insulator 3 b. The pusher pins 12are used to place a wafer 1 onto the RF electrode 4. When the wafer 1 istransported into the housing 7, the pusher pins 12 elevate and projectabove the RF electrode 4 to receive the wafer 1. Then, the pusher pins12 are lowered to place the wafer 1 onto the insulator 3 b.

[0009] The wafer 1 is securely held on the RF plate 4 by anelectrostatic chuck consisting of the insulator 3 b and the DC plate 2.A positive voltage of 1000V to 3000V is applied to the DC plate 2 fromthe DC power source 11. In this situation, if a radio frequency isapplied to the RF electrode 4 to produce plasma, the wafer 1 is chargedup negatively and attracted to the DC plate 2 that is at a positivevoltage. A low pass filter 13 prevents the RF power, which is applied tothe RF electrode 4 and transferred to the DC plate 2 via the parasiticcapacity between the RF electrode 4 and the DC plate 2, from flowinginto the DC power source 11.

[0010] In the conventional plasma processing apparatus shown in FIGS. 1and 2, as the radio frequency applied to the electrode is raised, lossdue to the inductance and the parasitic capacity of the hot linesbecomes large. This means that the loss of the RF power also increases.

[0011] The most significant example of loss due to the raised radiofrequency is the growth in parasitic capacity relative to the capacityof produced plasma. To be more precise, as the radio frequency becomeshigher, the plasma density increases, as illustrated in FIG. 4B. On thecontrary, the plasma capacity itself abruptly decreases as the frequencyincreases, as shown in FIG. 4A. For this reason, the parasitic capacityexisting between the RF supply line through which the radio frequencypropagates and the grounded portion of the housing 7 becomes almostequal to the plasma capacity (that is, the parasitic capacity increasesrelatively). This means that, apart from that used for producing andmaintaining plasma, the RF power supplied to the apparatus is wasted.

[0012] Another problem in the conventional plasma processing apparatusis the parasitic capacity existing between the RF mount electrode 4 andthe pusher pins 12. As is illustrated in FIG. 2, the conventional RFmount electrode 4 is made relatively thick because it is designed tofunction as both an electrode and a wafer mount stage. The pusher pins12 always face the metallic electrode 4 even after it retreats insidethe RF mount electrode 4, producing parasitic capacity between the pinsand electrode 4. This parasitic capacity causes a loss in RF power.

[0013] Such a loss becomes particularly marked if the applied radiofrequency is 60 MHz or higher. Accordingly, the reduction of parasiticcapacity is one of the most serious problems to be solved. In order toreduce the parasitic capacity, the entire apparatus, including the heatsink structure and pusher pin arrangement, must be configured optimally.

[0014] Still another problem in the conventional apparatus is the lossof RF power from the electrostatic chuck. As has been mentioned above,the radio frequency applied to the RF mount electrode flows into the lowpass filter 13 via the DC transmission line, and is consumed in thisfilter. This occurs because the low pass filter 13 consists oflumped-constant reactance elements, and has large parasitic capacity. Asthe radio frequency becomes higher, loss or the consumption in the lowpass filter 13 increases. The radio frequency flowing into the low passfilter 13 may cause the break down or burning of the low pass filter,and may damage the DC power source 11.

[0015] On the other hand, it is desirable to reduce the volume of thehousing 7 as much as possible in order to produce plasma efficiently,while reducing the quantities of precursor gases introduced into theapparatus. To reduce the volume of the apparatus, it has been proposedto use the heat sink (or insulator) that supports the RF electrode and awafer as a vacuum chuck itself. However, the interface between themetallic housing and the ceramic sink is located at the boundary betweenthe vacuum and the atmosphere. Consequently, the heat sink becomesbrittle and is likely to break.

SUMMARY OF THE INVENTION

[0016] Therefore, to overcome the problems in the prior art technique, aplasma processing apparatus that can reduce a loss of RF power even if aradio frequency of 60 MHz or higher is applied to the electrode isprovided in one aspect of the invention. In this apparatus, the plasmacapacity is increased relative to the parasitic capacity by reducing theparasitic capacity of the apparatus as a whole. This plasma processingapparatus comprises a grounded housing, a thin RF plate electrode placedin the housing, an opposite electrode facing the RF plate electrode, andan RF power source for applying a radio frequency to either the RF plateelectrode or the opposite electrode. By applying a radio frequency toeither electrode, plasma is produced between the RF plate electrode andthe opposite electrode. If the radio frequency applied to the electrodeis f (MHz), the parasitic capacity C (pF) between the grounded portionof the housing and a conductive portion through which the radiofrequency propagates is less than 1210*f^(−0.9).

[0017] In another aspect of the invention, a plasma processing apparatuscomprises a grounded housing, an RF plate electrode placed in thehousing, an opposite electrode facing the RF plate electrode, and firstand second radio-frequency power sources. The first and secondradio-frequency power sources apply different values of radiofrequencies to either the RF plate electrode or the opposite electrode.One of the radio frequencies applied to the electrode is 60 MHz orhigher. If this radio frequency is f(MHz), the parasitic capacity C (pF)between the grounded portion of the housing and a conductive portion (orhot lines) is also less than 1210*f^(−0.09).

[0018] In still another aspect of the invention, a plasma processingapparatus having an improved pusher pin structure for reducing theparasitic capacity is provided. In this apparatus, the parasiticcapacity between the RF plate electrode, to which a radio frequency isapplied, and pusher pins is substantially eliminated, thereby greatlyreducing the loss of RF power. This plasma processing apparatuscomprises a grounded housing, a wafer mount electrode having at leasttwo holes passing through it, an opposite electrode facing the wafermount electrode, an RF power source, and pusher pins inserted in theholes. The wafer mount electrode includes an RF plate electrode having athickness of 6 mm or less and an insulator for supporting the RF plateelectrode. The pusher pins are movable between a first position, atwhich the pusher pins project out of the wafer mount electrode toreceive a wafer, and a second position, at which the pusher pins retreatbelow the RF plate electrode during the generation of plasma.

[0019] In yet another aspect of the invention, a plasma processingapparatus that can reduce the loss of RF power, eliminate adverseinfluence to the DC power source, and produce plasma at a high density,is provided. This plasma processing apparatus comprises a wafer mountelectrode, an opposite electrode facing the wafer mount electrode, a DCpower source, and RF power source, and a radio-frequency trap positionedbetween the wafer mount electrode and the DC power source. The DC powersource supplies a direct-current voltage to hold a wafer on the wafermount electrode in an electrostatic manner. The RF power source appliesa radio frequency to either the wafer mount electrode or the oppositeelectrode to generate plasma between the two electrodes. Theradio-frequency trap has an electrical length of (2 n+1)/4 wavelength ofthe applied radio frequency, where n is 0 or a natural number.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Other objects and advantages will be apparent from the followingdetailed description of the invention in conjunction with the attacheddrawings, in which:

[0021]FIG. 1 illustrates a plasma processing apparatus;

[0022]FIG. 2 illustrates the positional relationship between the wafermount electrode and the pusher pins used in the conventional plasmaprocessing apparatus;

[0023]FIG. 3 illustrates a plasma processing apparatus according to thefirst embodiment of the invention;

[0024]FIG. 4A is a graph showing the plasma capacitance as a function offrequency applied to the plasma processing apparatus, and FIG. 4B is agraph showing the theoretical electron density and the actual electrondensity reflecting decrease in plasma capacitance in the higherfrequency range;

[0025]FIG. 5 illustrates a modification of the plasma processingapparatus shown in FIG. 3; and

[0026]FIG. 6 illustrates another modification of the plasma processingapparatus shown in FIG. 3;

[0027]FIG. 7 illustrates the positional relationship between the wafermount electrode and pusher pins according to the second embodiment ofthe invention;

[0028]FIG. 8A is a plan view of the wafer placed on the pusher pinsusing a folk, FIG. 8B is a cross-sectional view taken along the C-C lineof FIG. 8A, and FIG. 8C is a cross-sectional view of the wafer mountedon the wafer mount electrode with the pusher pins retreated inside theinsulating plate; and

[0029]FIG. 9 illustrates a plasma processing apparatus according to thethird embodiment of the invention, which uses an RF trap with anelectrical length of λ/4.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

[0030]FIG. 3 illustrates a plasma processing apparatus 100 according tothe first embodiment of the invention. The plasma processing apparatus100 has housing 107, a thin RF plate electrode 104 placed in thehousing, and an opposite electrode 105 facing the RF plate electrode104. The apparatus also has a heat sink 103 holding the RF plateelectrode 104, and an RF power source 109 for applying a radio frequencyto at least one of the RF plate electrode 104 and the opposite electrodeto produce plasma between these two electrodes. A matching box 131 isinserted between the RF power source 109 and the load to cancel thereactance component of the load and correct the impedance.

[0031] The RF plate electrode has a thickness of 6 mm or less, and morepreferably, 1 mm to 3 mm, which is relatively thin as compared with theconventional RF electrode. The thin RF plate electrode 104 is held onthe heat sink 103, which regulates the temperature of a wafer that is tobe processed. The heat sink 103 is, for example, a cylinder made ofceramics, and has a coolant passage 119. The coolant passage 119 islocated just under the RF plate electrode 104 to regulate thetemperature of a wafer 101 during plasma processing. To carry out plasmaprocessing with high precision, the wafer temperature must be keptuniform.

[0032] In addition to the coolant passage 119, a groove 120 is formed inthe heat sink 103.

[0033] By forming the coolant passage 119 and the groove 120, thedielectric constant of the heat sink 103 can be reduced as a whole, andconsequently, the parasitic capacity existing in the housing 107decreases. The coolant passage 119 and the groove 120 are annular orcurved.

[0034] The heat sink 103 is secured at the bottom edge on the shoulder125 of the housing 107. The diameter of the housing 107 becomes smallerat the shoulder 125. By mounting the heat sink 103 on the shoulder 125of the housing 107, a vacuum chamber is formed in the upper part of thehousing 107. Thus, the heat sink 103 functions as a vacuum chuck. Fromits bottom face, the heat sink 103 is in contact with the atmosphere.

[0035] In general, if the interface between the grounded housing 107,which is made of a conductive material, and the heat sink 103, which ismade of a ceramic material, is located at the boundary between thevacuum and the atmosphere, the heat sink is likely to break or crack. Toprevent such damage, a shock absorber 117 is inserted between the heatsink 103 and the housing 107. The shock absorber 119 is made of a softand malleable insulator, such as Teflon. By inserting a shock absorberbetween the housing (i.e., the conductor) and the heat sink (i.e., theinsulator, such as ceramics), the heat sink is protected from breakageeven if it is located at the boundary between the vacuum and theatmosphere.

[0036] As a feature of the plasma processing apparatus, if the radiofrequency applied to the RF plate electrode 104 is f (MHz), theparasitic capacity C (pF) existing in the housing is less than1210*f^(−0.9). By limiting the parasitic capacity to this value, theplasma capacity is increased relative to the parasitic capacity, and asa result, the plasma density can be increased.

[0037] This concept will be explained with reference to FIGS. 4A and 4B.FIG. 4A is a graph of the plasma capacity (pF) as a function of theradio frequency f (MHz) applied to the RF plate electrode 104 shown inFIG. 3. As is clearly shown in the graph, as the radio frequency appliedto the electrode is raised, the plasma capacity abruptly decreases. Therelation between the plasma capacity and the applied frequency is

C(pF)=1209.9×f ^(−0.9016)   (1)

[0038] It is generally convinced that as the applied radio frequencybecomes higher, the electron density in the produced plasma increases.The general understanding was that electron density is in proportion tothe square of the applied frequency, and therefore, it was believed thatplasma processing could be carried out more efficiently by theapplication of higher RF power.

[0039] However, in reality, the produced plasma capacity decreases athigher frequency, as is shown in FIG. 4A. If the parasitic capacity ofthe circuit (that is, the capacity generated between the ground and theconductive passes through which the radio frequency propagates) is, forexample, about 10 pH as indicated by the dashed line C in FIG. 4A, then,the produced plasma capacity becomes almost equal to the parasiticcapacity if the applied radio frequency is raised up to 200 MHz. Thismeans that the applied radio frequency is wasted on irrelevant activityother than producing plasma. If the radio frequency is further raised,the produced plasma capacity falls below the dashed line C, which meansthat the parasitic capacity is greater than the produced plasmacapacity.

[0040]FIG. 4B is a graph of electron density as a function of appliedfrequency in a conventional parallel plate plasma etching apparatusdesigned for 13.56 MHz. The curved line L1 represents the theoreticalvalue of the electron density, which is in proportion to square ofapplied frequency (f²). The line L2 represents the actual electrondensity taking the decrease of produced plasma capacity into account. Ashas been explained in conjunction with FIG. 4A, the plasma capacitydecreases in proportion to f^(−0.9). Accordingly, the plasma density(i.e., the electron density) increases in proportion to power of 1.1 ofapplied frequency.

[0041] In order to guarantee that the increase in electron density is atleast in proportion to f^(1.1), the apparatus must be designed so thatthe parasitic capacity C is kept under 121×f ^(−0.9016) (pF) even if theapplied radio frequency is raised.

[0042] The parasitic capacity exists between the hot lines through whichthe radio frequency propagates and the grounded portion. The hot linesinclude, for example, the conductive extending from the RF power source109 to the RF plate electrode 104, and the RF plate electrode itself. Toreduce the parasitic capacity, the following measures are taken in theplasma processing apparatus of the first embodiment:

[0043] (i) selecting a coolant that has as low a dielectric constant aspossible (or at least lower than the dielectric constant of the heatsink103);

[0044] (ii) forming a groove or a capacity in the heat sink 103; and

[0045] (iii) setting the dimensions of the elements of the apparatusoptimally.

[0046] Item (iii) relates to the gap “a” between the opposite electrode105 and the wafer surface, the space “b” between the edge of the RFplate electrode 104 and the housing 107, the distance “c” from theshoulder 125 to the RF plate electrode 104, and spaces “e” and “f” fromthe conductive lines to the housing 107 being set optimally.

[0047] For example, a 300 mm wafer is processed in the apparatus, andthe gap “a” between the opposite electrode 105 and the surface of thewafer 101 is set to 20 mm. Under this condition, it is desirable to havea large space “b” between the edge of the RF plate electrode 104 and thehousing 107 in order to reduce the parasitic capacity produced in thisportion. However, the space “b” cannot be increased too freely becausethe apparatus would become huge. Therefore, the space “b” is set toabout twice of “a”, that is, about 40 mm. The distance “c” from thebottom of the heat sink 103 to the RF plate electrode 104 is set to b*εr (where ε r is the relative dielectric constant of the heat sink 103).

[0048] The parasitic capacity C between the RF plate electrode 104 andthe shoulder 125 of the housing 107, at which the heat sink 103 is held,is given by ε S/c (where ε is the dielectric constant of the heat sink103 and S is the contact area between the bottom of the heat sink 103and the shoulder 125 of the housing 107). If the distance “c” is set toosmall, the parasitic capacity becomes large. If the distance “c” is setlarge, the apparatus become large. For this reason, “c” is set equal tob*ε_(r), in order to guarantee the mechanical strength of the heat sink103 even allowing for the inclusion of the coolant passage 119 and thegroove 120, while preventing the apparatus from becoming too large.

[0049] If the heat sink 103 is made of quartz, the relative dielectricconstant ε r is about 6. If the heat sink 103 has no coolant passages119 or grooves 120, the distance “c” is 240 mm based on the calculationof b*ε_(r). Under these conditions, the dielectric constant of theentirety of the heat sink 103 can be reduced by:

[0050] (i) Making the volume of the groove 120 as large as possiblewithin a range not adversely affecting the mechanical strength of theheat sink 103;

[0051] (ii) Using a coolant that has as low a dielectric constant aspossible; and

[0052] (iii) Combining (i) and (ii).

[0053] Concerning item (i), the relative dielectric constant in thegroove is about 1, and therefore, as the groove becomes large, thedielectric constant of the heat sink 103 as a whole becomes smaller.Concerning item (ii), an example of the coolant is fluorine-containinginert liquid, such as Fluorinert (manufactured and sold by Sumitomo 3MCo. Ltd.) whose dielectric constant is about 2.5. The coolant passage islocated in proximity to the wafer in order to regulate the temperatureof the wafer 101.

[0054] It is also necessary to make the contact area S between thebottom of the heat sink 103 and the shoulder 125 of the housing 107small in order to reduce the parasitic capacity. However, if the contactarea S is fixed small, the interface between the metallic housing andthe brittle heat sink will be located very close to the boundary betweenthe vacuum and the atmosphere. This causes the heat sink 103 to beeasily breakable. To overcome this problem, a shock absorber 117 isinserted between the bottom of the heat sink 103 and the shoulder of thehousing 107. In this arrangement, the parasitic capacity can be reducedwithout damaging the heat sink 103. Consequently, plasma can be produceduniformly above the wafer at a high density, while minimalizing the lossof RF power.

[0055]FIG. 5 illustrates a modification of the plasma processingapparatus shown in FIG. 3. In the example of FIG. 5, two RF powersources are used, and two different radio frequencies are applied to theelectrode. Matching boxes 231 and 232 are also provided in parallel. Thefirst RF power source 209 supplies a radio frequency of about 60 MHz to100 MHz to generate plasma, and the second RF power source 210 suppliesa radio frequency of about 1 MHz to adjust the ion energy. By regulatingthe voltages of these power sources, the plasma density and the ionenergy can be balanced. To be more precise, the second RF power source210 is regulated to control the etching rate and the pattern formation.By reducing the ion energy, the plasma processing apparatus 200 can beused for CVD, for example, filling VIA holes.

[0056] A coolant passage 219 is formed in the heat sink 203 in proximityto the wafer 201, and a groove 220 is formed so as to be as large aspossible within a range that does not adversely affect the mechanicalstrength of the heat sink 203. The dimensions a, b, c, e, and f areselected optimally, as in the example shown in FIG. 3. The contact area(or the interface) between the heat sink 203 and the housing 207 isfixed small, and a shock absorber 217 is inserted in this portion toprotect the heat sink 203. With this arrangement, both the ion energyand the plasma density are controlled simultaneously, while reducing theparasitic capacity efficiently, and highly precise plasma processing canbe carried out.

[0057]FIG. 6 illustrates another modification of the plasma processingapparatus shown in FIG. 3. In this example, a cavity 330 is formed inaddition to the coolant passage 319, and the matching box 331 and afilter are accommodated in the cavity 330. The cavity 330 contributes toreducing the dielectric constant of the heat sink 303 as a whole, andconsequently, the parasitic capacity of the apparatus is reduced. Inaddition, the cavity 330 allows the apparatus to be made compact.

[0058] The heat sink 303 is supported on the on a part of the housing307 with a cushion 317 inserted between the heat sink 303 and thehousing 307. Again, the heat sink 303 itself functions as a vacuum chuckto define a sealed space for producing plasma, while cracks or breakageof the heat sink 303 that is located at the boundary between the vacuumand the atmosphere is prevented.

Second Embodiment

[0059]FIG. 7 illustrates a wafer mount electrode 500 and pusher pinspenetrating the wafer mount electrode. As has been explained above, ifthe plasma processing is carried out at a higher range of radiofrequency, the parasitic capacity of the apparatus has to be reduced asmuch as possible in order to minimalize the loss of RF power. Thecapacitive coupling between the electrode and the pusher pins is one ofthe more significant factors regarding the parasitic capacity in theconventional plasma processing apparatus.

[0060] In the prior art, as illustrated in FIG. 2, the pusher pins 12are elevated in the through-holes 15 to the position B when mounting toor removing a wafer from the wafer mount electrode. During thegeneration of plasma, the pusher pins 12 are retracted to the position Ainside the RF electrode 4 in the conventional apparatus. The loss of theRF power due to the capacitive coupling between the pusher pins 12 andthe RF electrode 4 is significant, and cannot be neglected. Because thepusher pins 12 play an important role in discharging residual charges,in addition to receiving the wafer, the pins 12 cannot be replaced withinsulating pins. For this reason, the parasitic capacity of the pusherpins 12 that is capacitively coupled with the RF electrode 4 has beenthe main cause of the loss of the RF power.

[0061] Increasing the gap d between the pusher pin 12 and the RFelectrode 4 can reduce the capacitive coupling between the pusher pinand the electrode. However, the distance from the inner face of thethrough-hole 15 to the pusher pin 12 has to be kept to 0.8 mm or less toprevent abnormal discharge due to plasma entering into the gap. Thisrequirement prevents the reduction of the parasitic capacity C=ε S/d,where S is the surface area of the pusher pin 12 that faces the innerface of the through-hole 15. The area S is defined by the diameter andthe height L of the pusher pin 12.

[0062] The conventional RF electrode has a thickness of about 15 mm.Meanwhile, it is desirable that the vertical stroke of the pusher pin isset small, preferably, at 8 mm or less. Accordingly, it was difficultfor the conventional structure to reduce the parasitic capacity betweenthe pusher pin and the electrode.

[0063] To overcome this problem, the wafer mount electrode 500 of thesecond embodiment is comprised of a thin RF plate electrode 504, and aninsulating plate 506 to reinforce the RF plate electrode 504, asillustrated in FIG. 7. The thickness of the RF plate electrode 504 is 6mm or less, and 3 mm of thickness is selected in the preferredembodiment. This arrangement efficiently prevents the pusher pin 502from being capacitively coupled with the wafer mount electrode 500during the generation of plasma. This wafer mount electrode can besuitably used in the plasma processing apparatuses shown in FIGS. 3, 5and 6.

[0064] The insulating plate 506 is bonded to the RF plate electrode 504.The wafer mount side of the RF plate electrode 504 is covered with aninsulator 503, and a DC plate 502 is placed inside the insulator 503 toprovide an electrostatic chuck. The insulator 503 is made of the samematerial as the heat sink used in the plasma processing apparatusesshown in FIGS. 3, 5 and 6. The insulating plate 506 supporting the RFplate electrode 504 may be of the same material as the insulator 503, oralternatively, of a different material from the insulator 503.

[0065] The thickness of the RF plate electrode 504 of the wafer mountelectrode 500 is greatly reduced, as compared with the metallic portionof the conventional wafer mount electrode. Two or more through-holes 505are formed in the wafer mount electrode 500, and pusher pins 502 made ofa conductor or semiconductor are inserted in the through-holes 505. Thepusher pin 502 is driven by a driving mechanism (not shown) between theupper position B and the lower position A indicated by the dashed linesin FIG. 7. When mounting to and removing a wafer from the wafer mountelectrode 500, the pusher pins 502 project out from the wafer mountelectrode 500 up to position B. The pusher pins 502 are retracted toposition A, and are positioned below the RF plate electrode 504 duringthe generation of plasma. In position A, the pusher pins 502 face theinsulating plate.

[0066]FIG. 8 illustrates the vertical displacement of the pusher pins502 more clearly. A wafer 601 is placed on a fork 606 and transportedinto the plasma processing apparatus shown in FIG. 3, 5 or 6, asillustrated in FIG. 8A. Then, the wafer 601 is positioned over the wafermount electrode 500. At this time, the pusher pins 502 project from thethrough-holes 505 and receive the wafer 601, as illustrated in FIG. 8B.Upon placing the wafer 601 onto the pusher pins 502, the fork 606 leavesthe chamber (not shown).

[0067] Then, the pusher pins 502 are retracted into the through-hole andremain below the RF plate electrode 504, as illustrated in FIG. 8C. Thewafer 601 is held on the wafer mount electrode 500 in an electrostaticmanner. Then, a radio frequency is applied to the RF plate electrode 504to start plasma processing. Since the pusher pins 502 are positionedbelow the RF plate electrode 504 and face the insulating plate 506,capacitive coupling between the pusher pin 502 and the RF plateelectrode can be avoided.

[0068] In the second embodiment, the stroke of the pusher pins 502 isonly 6 mm because the thickness of the RF plate electrode is reduced to3 mm. This means that the pusher pin 502 is distanced from the RF plateelectrode 504 by about 3 mm. Consequently, the value “d” increasessubstantially, and the parasitic capacity between the pusher pin 502 andthe RF plate electrode 504 is reduced greatly.

[0069] Preferably, the stroke of the pusher pin 502 is 8 mm or smaller.If the stroke is set to the maximum (i.e., 8 mm), the pusher pin 502 isfurther away from the RF plate electrode 504, and the parasitic capacityis further reduced. In any cases, it is preferable that the stroke ofthe pusher pins between the first and second positions is twice or moreof the thickness of the RF plate electrode. Because a thin RF plateelectrode 504 is used, the stroke of the pusher pin 502 can be reduced.At the same time, the pusher pin 502 retreats completely into theinsulating plate 506 below the RF plate electrode 504, and the.parasitic capacity between the pusher pins 502 and the electrode 504 canbe eliminated during the generation of plasma. With this arrangement,the parasitic capacity C (pF) between the grounded portion of thehousing and hot lines is again less than 1210*f^(−0.9).

[0070] Although, in the second embodiment, the thickness of the RF plateelectrode 504 is set to 3 mm, any value may be selected from a rangebetween 1 mm to 6 mm, taking into account the desired stroke and thethickness of the insulator 503 on the RF plate electrode 504. Withinthis range, the parasitic capacity can be reduced efficiently.

Third Embodiment

[0071]FIG. 9 illustrates a parallel plate plasma processing apparatus700 according to the third embodiment of the invention. The apparatushas a heat sink (or an insulator) 703 for mounting a wafer 701, a DCplate 702 positioned near the surface of the heat sink 703, an RF plateelectrode 704 extending below the DC plate 702, and an oppositeelectrode 705 facing the RF plate electrode. The DC plate 702, the heatsink 703, and the RF plate electrode 704 comprise a wafer mountelectrode.

[0072] The plasma processing apparatus 700 also has a DC power source711 for applying a DC voltage to the DC plate 702 to hold a wafer 701 inan electrostatic manner, and an RF power source 709 for applying a radiofrequency to the RF plate electrode 704. Plasma is produced between theRF plate electrode 704 and the opposite electrode 705 by application ofthe radio frequency.

[0073] The feature of the third embodiment is a radio frequency trap(referred to as an RF trap) 715 having an electrical length of ¼ of thewavelength of the applied radio frequency (e.g., 100 MHz in the thirdembodiment), which is inserted between the DC plate 702 and the DC powersource 711. One end of the RF trap 715 is connected to the DC plate 702,and the other end is connected to a 1000 pF bypass capacitor 718 and theDC power source 711.

[0074] The RF trap 715 having an electrical length of ¼ wavelength ofthe applied radio frequency, prevents radio frequency from flowing intothe DC power source side from the DC plate, thereby eliminating adverseaffects on the DC power source 711.

[0075] For example, if the applied radio frequency is 100 MHz and thevelocity is equal to the speed of light (c), the λ/4 is obtained fromthe following equations.

F*λ=c

λ/4=c/4f=3*10⁸ m/(4*100*10⁶)=0.75 m

[0076] In practice, the radio frequency propagates through a conductivematerial, and therefore, the value of λ/4 is shorter than 0.75 m. In thethird embodiment, the physical length of the RF trap 715 is 27 cm. TheRF trap 715 is a silver-plated copper pipe with an outer diameter of 1cm. By using a copper pipe, the radio frequency component can beefficiently trapped without preventing the flow of the direct currentapplied to the DC plate 702.

[0077] The above-mentioned physical length of the RF trap 715 isdetermined using an oscilloscope. The oscilloscope is connected to theRF plate electrode 704, and the length of the pipe is adjusted tomaximum amplitude. The physical length of 27 cm is shorter than thetheoretical electrical length of λ/4 (which is v/4 f, where v denotesthe propagation velocity in the conductor). This is because of theparasitic capacity that exists between the hot lines and the groundedportion of the housing.

[0078] The plasma density was measured in the apparatus using the RFtrap 715 of the third embodiment, and compared with that produced in theconventional apparatus.

[0079] In the experiment, 1500V DC voltage was applied to the DC plate2, and Ar gas was introduced from the gas port 6 into the conventionalapparatus shown in FIG. 1. The pressure in the housing 7 was kept at 10Pa. Then, 1 kW RF power was applied to the RF electrode 4 to produceplasma. The electron density of the plasma was measured by a phaseinterferometer for transmission microwave. The measured density was4*10¹¹ cm⁻³.

[0080] The same measurement was conducted in the plasma processingapparatus 700 with the RF trap 715 under the same conditions. Themeasured plasma density was 5*10¹¹ cm⁻³. The plasma density increases by20% in the apparatus 700 of the third embodiment, as compared toconventional apparatus. The increase in plasma density can solve theproblem of the prior art, that is, the loss of RF power. In other words,the ratio of the RF energy used for generation of plasma to the entireRF energy applied to the apparatus has increased, and high-speed andhigh-precession plasma processing can be realized.

[0081] Although, in the third embodiment, the RF trap 715 is placedbefore the DC power source 711 used for electrostatic chuck, another RFtrap may be positioned before another DC power source used otherpurposes. The RF trap 715 can be inserted in the conductive line betweenthe RF plate electrode and the chalk coil of the plasma processingapparatus of the first embodiment (FIGS. 3, 5 and 6). In this case, theelectrical length of the RF trap is again λ/4 of the applied radiofrequency.

[0082] The electrical length of the RF trap is not limited to λ/4. Theelectrical length may be set to 3 λ/4, 5 λ/4, . . . (2 n+1) λ/4, as longas the RF trap can reflect the radio frequency component at the peak ofits amplitude. By inserting the trap having these electrical lengths,the radio frequency transmitted from the wafer mount electrode isreflected off the trap and prevented from flowing into the DC powersource. Because the DC power source is protected from damages, a loss ofRF power is reduced. Consequently, high-density plasma is produced inthe apparatus. In this case, the physical length of the radio-frequencytrap is set at less than (2 n+1)/4 wavelength of the applied radiofrequency taking into account the parasitic capacity of the plasmaprocessing apparatus and the inductance of transmission lines.

[0083] The RF trap may be made of any good conductors, other thancopper. The bypass capacitor connected in parallel to the DC powersource supplies a DC voltage to the RF trap, and has sufficiently lowimpedance with respect to the radio frequency. The RF voltage becomeszero at the end of the RF trap connected to the bypass capacitor, andtherefore, no RF voltage adversely affects the CD power source.

[0084] As has been described above, the plasma processing apparatus ofthe present invention is configured so that the parasitic capacitybetween the hot lines through which the radio frequency propagates andthe grounded portion of the housing is less than 1210*f^(−0.9).Consequently, the plasma capacity increases relatively, and high-densityplasma processing can be achieved.

[0085] A portion of the housing supports the bottom of the heat sink,and the heat sink itself functions as an electrostatic chuck. In thisarrangement, a shock absorber is inserted between the heat sink and thehousing, thereby preventing breakage or cracks of the heat sink at theboundary between the vacuum and the atmosphere. Thus, a compact and asafe plasma processing apparatus can be realized.

[0086] The thickness of the RF plate electrode is greatly reduced, andthe positional relationship between the RF plate electrode and thepusher pins is improved so as to substantially eliminate the capacitivecoupling between the RF plate electrode and the pusher pins.

[0087] The RF trap also prevents undesirable waste (or loss) of the RFpower, and eliminates adverse influence to the DC power source.

[0088] The present invention is not limited to the above-describedexamples, and there are many possible modifications and substitutions.For example, the wafer includes any equivalent objects be processed inthe plasma processing apparatus of the present invention, and the heatsink may be made of any materials that is a good insulator and a goodheat radiator. Plasma processing includes not only plasma etching, butalso film formation, such as CVD. The plasma processing apparatus of thepresent invention can also be applied to plasma ion implantation.

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 10. A plasma processing apparatus comprising: a groundedhousing; an RF plate electrode placed in the housing; an oppositeelectrode facing the RF plate electrode; and first and secondradio-frequency power sources for applying different values of radiofrequencies to either the RF plate electrode or the opposite electrode,one of the radio frequencies being 60 MHz or higher, which isrepresented as f(MHz), wherein the parasitic capacity C (pF) between thegrounded portion of the housing and a conductive portion through whichthe radio frequencies propagate is less than 1210*f^(−0.9).
 11. Theplasma processing apparatus of claim 10, further comprising a heat sinkthat holds the RF plate electrode and has a coolant passage in proximityto the RF plate electrode.
 12. The plasma processing apparatus of claim11, wherein the heat sink further has a groove inside it.
 13. The plasmaprocessing apparatus of claim 11, wherein the heat sink further has acavity.
 14. The plasma processing apparatus of claim 11, wherein theheat sink is supported by a part of the housing, and a shock absorber isinserted between the heat sink and said part of the housing.
 15. Theplasma processing apparatus of claim 14, wherein the heat sink is madeof a ceramic material and the shock absorber is a malleable insulator.16. The plasma processing apparatus of claim 14, wherein the shockabsorber is made of Teflon.
 17. A plasma processing apparatuscomprising: a grounded housing; a wafer mount electrode placed in thehousing and having at least two holes penetrating it, the wafer mountelectrode including an RF plate electrode having a thickness of 6 mm orless and an insulator for supporting the RF plate electrode; an oppositeelectrode facing the wafer mount electrode; an RF power source forapplying a radio frequency to either the wafer mount electrode or theopposite electrode to generate plasma between the two electrodes; andpusher pins inserted in the holes and movable between a first position,at which the pusher pins project out of the wafer mount electrode inorder to receive a wafer, and a second position, at which the pusherpins retreat below the RF plate electrode during the generation ofplasma.
 18. The plasma processing apparatus of claim 17, wherein thestroke of the pusher pins between the first and second positions istwice or more of the thickness of the RF plate electrode.
 19. The plasmaprocessing apparatus of claim 17, further comprising a heat sink forholding the wafer mount electrode, wherein the insulator is a part of orthe entirety of the heat sink, and the heat sink has at least one of acoolant passage, a groove, and a cavity.
 20. The plasma processingapparatus of claim 19, wherein the heat sink is supported by a part ofthe housing, and a shock absorber is inserted between the heat sink andsaid portion of the housing.
 21. The plasma processing apparatus ofclaim 17, wherein if the radio frequency applied to the electrode is f(MHz), the parasitic capacity C (pF) between the grounded portion of thehousing and a conductive portion through which the radio frequencypropagates is less than 1210*f^(−0.9).
 22. The plasma processingapparatus of claim 17, wherein the pusher pins are made of a conductoror a semiconductor.
 23. A plasma processing apparatus comprising: awafer mount electrode; an opposite electrode facing the wafer mountelectrode; a DC power source for supplying a direct-current voltage inorder to hold a wafer on the wafer mount electrode in an electrostaticmanner; an RF power source for applying a radio frequency to either thewafer mount electrode or the opposite electrode to generate plasmabetween the two electrodes; and a radio-frequency trap positionedbetween the wafer mount electrode and the DC power source, theradio-frequency trap having an electrical length of (2 n+1)/4 wavelengthof the applied radio frequency.
 24. The plasma processing apparatus ofclaim 23, wherein the radio-frequency trap is a conductive pipe.
 25. Theplasma processing apparatus of claim 23, wherein the physical length ofthe radio-frequency trap is set shorter than (2 n+1)/4 wavelength of theapplied radio frequency taking into account parasitic capacity of theplasma processing apparatus and inductance of transmission lines. 26.The plasma processing apparatus of claim 23, further comprising a bypasscapacitor connected in parallel with the DC power source; wherein oneend of the radio-frequency trap is connected to the wafer mountelectrode, and the other end of the radio-frequency trap is connected tothe DC power source and the bypass capacitor.
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